The field of the present invention relates to light sources. In particular, a light source for a passive optical network is disclosed herein.
This application is related to subject matter disclosed in commonly owned (i) U.S. provisional App. No. 60/739,398 entitled “Laser source for passive optical network” filed Nov. 23, 2005 in the name of Henry A. Blauvelt, (ii) U.S. non-provisional application Ser. No. 11/562,684 entitled “Laser source for a passive optical network” filed Nov. 22, 2006 in the name of Henry A. Blauvelt, and (iii) international App. No. PCT/US2006/061235 entitled “Laser source for a passive optical network” filed Nov. 23, 2006 in the name of Henry A. Blauvelt and published May 31, 2007 as WO 2007/062407. Each of said applications is hereby incorporated by reference as if fully set forth herein.
In a typical passive optical network (PON), multiple network termini 102 are connected to a single network node 104 via an optical splitter network 106 (FIG. 1). Downstream optical signals are routed from the network node 104 through the splitter network 106 and reach all of the network termini 102, regardless of the intended target terminus of the signal. Upstream signals originating at a network terminus 102 are transmitted through the splitter network 106 to the network node 104. If upstream optical signals are transmitted from two or more network termini 102 simultaneously, those signals might interfere with one another upon reaching the network node 104, or further upstream from the network node.
Upstream optical signals are typically generated at a network terminus in response to radio-frequency (RF) electrical signals carrying desired information. The upstream optical signal typically comprises some DC optical power level (also referred to as a DC optical set point), with the information carried by optical modulation about the DC optical power level set point. The modulation of the optical signal is typically driven by the corresponding RF electrical signal. The source of the optical signal is typically a semiconductor laser source (e.g., a diode laser such as a Fabry-Perot [FP] laser, a distributed Bragg reflector [DBR] laser, or a distributed feedback [DFB] laser), but any suitable coherent or incoherent optical source can be employed (e.g., a light-emitting diode [LED] or a non-semiconductor optical source).
It may be desirable to provide a light source for use in a passive optical network that reduces the likelihood or severity of interference between simultaneously transmitted upstream optical signals.
In the commonly owned applications listed above, e.g., application Ser. No. 11/582,684 (the '684 application), the likelihood of interference between simultaneously transmitted upstream optical signals is decreased (i) by substantially reducing optical power emitted by a laser source when it is not receiving any electronic modulation signal, and (ii) by limiting emitted optical power to a DC optical power level just sufficient to accommodate the modulation imposed by an electronic RF modulation signal. An exemplary laser drive and modulation circuit disclosed in the '684 application is shown in FIG. 2 and its corresponding operational behavior is illustrated schematically in FIG. 3.
In FIG. 2, an incoming RF electrical signal 430 is split by RF tap 412. A fraction of the RF signal 430 reaches RF detector 410, which is operatively coupled to laser power control circuit 408. A laser current control 402b comprises any suitable circuit for controlling laser drive current through laser diode 402a (the circuit 402b shown in FIG. 2 is exemplary only) and is arranged to provide a DC laser power level that varies approximately linearly with an applied control voltage Vcontrol that is provided by the laser power control circuit 408. The remaining fraction of the RF signal 430 is coupled in any suitable way directly to laser diode 402a for modulating the laser output power, in this example through an impedance-matching component or network 411 and a capacitor network C1 and C2 in these examples. The total laser diode current is the sum of the DC current controlled by laser current control 402b and the RF signal applied to the laser diode 402a. The direct coupling of the RF signal to the laser diode comprises the modulator or modulating means in the exemplary embodiments; any other suitable modulator or modulating means can be employed. RF detector 410 produces a detector voltage VRF approximately proportional to the detected RF signal level, which can be detected RF signal amplitude or detected RF signal power. The impedance-matching component or network 411 typically is employed for matching the low impedance of the laser diode 402a to the impedance characteristics of the RF electrical signal transmission system that transmits RF signal 430 (e.g., 75 ohms for a typical coaxial cable system). Component or network 411 can comprise one or more resistors, one or more transformers, or any other suitable component or network for achieving the desired impedance-matching functionality; no specific configuration for network 411 is disclosed in the '684 application.
Laser current control 402b in this exemplary embodiment comprises a bias control circuit that varies the DC laser drive current allowed to flow through the laser diode 402a. The laser diode 402a is forward-biased by laser bias voltage Vlaser. The DC current allowed to flow through the laser diode 402a varies according to Vcontrol and a monitor voltage Vmon produced by monitor photodiode 416 (which receives a portion of the laser output power 420 and is reverse-biased by VPD in this example). Vcontrol serves as the DC set point control voltage, and the circuit 402b acts to maintain the laser output power (as reflected by Vmon) at the laser power DC set point. The embodiment of laser current control 402b is only one example of myriad circuits or components that can be employed within the scope of the present disclosure for controlling the DC laser output power.
The exemplary configuration shown in FIG. 2 for power control circuit 408 exhibits a dependence of a laser control voltage Vcontrol (and hence the laser power DC set point) on the RF detector voltage VRF substantially as shown in FIG. 3. When VRF from RF detector 410 is below a selected RF threshold voltage Vthr, the power control circuit 408 supplies a low-level control voltage to the laser current control 402b. This low-level voltage results in a low-power idle level for the output of the laser diode 402a. Diode D1 causes the laser control voltage Vcontrol to abruptly increase to the turn-on voltage of diode D1 as VRF increases through Vthr. When VRF exceeds the selected threshold voltage Vthr, the voltage supplied by the power control circuit 408 to the laser current control 402b varies substantially linearly with the RF detector voltage VRF over a selected range of VRF from about Vthr to a saturation input voltage VRF-S. At the saturated input voltage, the corresponding saturated control voltage is Vcontrol-S. The slope of the linear portion of the dependence of Vcontrol on VRF typically is substantially determined as known in the art by the operating characteristics of amplifier A1 (typically comprising one or more operational amplifiers with suitable feedback circuitry or components) and the values of one or more of the resistors R1-R5 (and may also depend on other circuit elements not shown in FIG. 2). A desired slope can be obtained by suitable adjustment of any one or more of those components. The saturation voltages typically are also determined in part by those components and can also depend on a supply voltage used to power the power control circuit 408.
The power control circuit can be operated so that when little or no RF signal is present at the RF detector 410, the laser output power is kept at a low level or turned off. When implemented at multiple network termini, this reduces the overall amount of laser power propagating upstream through the splitter network of the passive optical network, thereby reducing the likelihood or the severity of interference between upstream optical signals originating from different network termini. The power control circuit 408 can be arranged for turning off the laser when no RF signal is present or when the RF signal is below a selected threshold signal level (e.g., by reducing laser diode current below the lasing threshold). Alternatively, it may be desirable for the laser to remain above the lasing threshold but at a reduced idle power when the RF signal is absent or below the threshold signal level, e.g., so that the turn-on time for the laser or its controller might be shorter when an RF signal does appear. The threshold voltage Vthr can be selected so that the laser remains at its low-level idle power in the presence of stray sources of RF interference, but rises to a higher transmitting laser power DC set point when an RF electrical signal exceeding the RF threshold signal level reaches the RF detector. Depending on necessary or desirable performance characteristics for the laser power control circuit 408, Vthr can be set approximately at zero, or can be set at any suitable non-zero value.
The laser power DC set point should be sufficiently large so that modulations thereof by the RF signal are not clipped or otherwise distorted. It may also be desirable, however, to limit overall laser output power so as to reduce the likelihood or severity of interference between independent upstream optical signals in a passive optical network. This is achieved in the example of FIG. 2 by increasing the laser power DC set point monotonically with respect to an increasing detected RF signal level. Substantially linear or substantially proportional variation of the laser power DC set point with respect to the detected RF amplitude or power can be employed, for example, when the detected RF signal level exceeds a selected RF threshold signal level. In the exemplary power control circuit 408 shown in FIG. 2, Vcontrol varies substantially linearly with VRF over the selected operational range from about Vthr to about VRF-S. A desired slope may be selected by suitable choice of amplifier A1 (or its components) and one or more of the resistors R1-R5 to limit the overall transmitted optical power to only enough to ensure that the laser power DC set point is sufficiently high for a given RF modulation level.
Other types of optical sources can be employed, and other types of modulation of the optical source can be employed. Other circuit types or circuit configurations can be employed for providing the functionality of power control circuit 408 or laser current control 402b. Any suitable substantially monotonic dependence of the DC optical output power level versus the detected RF signal level may be employed, including substantially proportional variation, substantially linear variation, or other substantially monotonic variations.
While reducing the likelihood or severity of interference between simultaneously transmitted upstream optical signals in a passive optical network, the exemplary embodiment disclosed in the '684 application and shown in FIGS. 2 and 3 can exhibit certain performance characteristics that are undesirable in some circumstances. For example, in the low-RF state one or more components (e.g., an operational amplifier) of the amplifier A1 in FIG. 2 can be driven to a power supply rail voltage. When an RF modulation signal then appears at RF detector 410, there is often a significant lag time (often on the order of several microseconds) before the amplifier A1 responds with a laser control voltage that tracks the detected RF power. In some telecommunications applications, that lag time is unacceptably long and can result in lost data or a disrupted telecommunication link. In another example, RF signal leakage through RF detector 410 and laser power control circuit 408 into laser current control 402b can introduce distortion into the RF-modulated optical signal produced by laser diode 402a. 
It may be desirable to provide a light source for use in a passive optical network (i) that reduces the likelihood or severity of interference between simultaneously transmitted upstream optical signals, (ii) that reduces lag time between appearance of an RF modulation signal and response of an optical power control circuit (sub-microsecond response times may be desirable), (iii) that reduces or eliminates RF distortion of an RF-modulated optical signal arising from RF leakage through an RF detector and an optical power control circuit, or (iv) that reduces or eliminates leakage of transient electrical signals from an optical power control circuit or current control into an RF detector or into an RF source or transmission system.